The present invention relates to a system for controlling an air/fuel ratio of a fuel mixture supplied to an internal combustion engine.
In order for precise control of the air/fuel ratio, the feedback control systems employing an oxygen sensor are widely used in the automotive industry. In the feedback control systems, the amount of fuel required by the engine is precisely calibrated in accordance with input data involving the sensor output of the oxygen sensor indicative of the oxygen concentration in exhaust gases.
The typical feedback control systems for precise control of the air/fuel ratio are briefly discussed hereinafter.
(I) Feedback control to the stoichiometry:
In this feedback control, a correction coefficient is determined in response to the output signal of an oxygen sensor installed to probe the exhaust gases. With this correction coefficient, the basic amount of fuel to be supplied to the engine is corrected, so the actual air/fuel ratio is closely adjusted to the stoichiometry. The oxygen sensor employed by this known system cannot detect the air/fuel ratio of the rich fuel mixture. This system is described in a technical paper entitled "ECCS L-series Engine" published in June 1981 by Nissan Motor Company Limited.
(II) Feedback to the air/fuel ratio of the lean fuel mixture:
Laid-open Japanese Patent Application No. 56-89051 discloses a feedback control system which employs an oxygen sensor for precise control of the air/fuel ratio of the lean fuel mixture. Although it can detect the air/fuel ratio of the lean fuel mixture over a wide range, this oxygen sensor can not detect the air/fuel ratio of the rich fuel mixture over a wide range.
(III) Feedback control with a learning mode:
Laid-open Japanese Patent Application No. 58-124032 discloses a control system which has the learning mode to determine an appropriate input data for feedback control for the subsequent use for a feedforward control under operating condition where the output of an oxygen sensor is not relied upon, for example, for engine cranking.
In the above discussed known control systems, the feedback control is clamped so as to enrich the fuel mixture to operate the engine on the rich fuel mixture for engine warming-up, heavy (full) load, and transients. Because the conventional oxygen sensor cannot detect the air/fuel ratio of the rich fuel mixture over a wide range, the control precision of the air/fuel ratio has been decreased under operating conditions of the engine where the rich fuel mixture is required, resulting in deterioration in driveability if the actual mixture is leaner than desired or increased exhaust emissions if the actual mixture is richer than desired.